the coking properties of coal at elevated pressures. - Argonne ...

under which conditions sulfur species generated during the combustion of pulverized
coal can be captured and retained by calcium containing sorbents. Two series of
experiments were carried out: one in which any capture would take place primarily
under oxidizing conditions and the other in which significant residence times in
the rich zone would allow capture under reducing conditions. Under oxidizing conditions
the thermal environment experienced by the sorbent particle appears to be the
dominant parameter controlling sulfur capture. This is probably because of deadburning.
If a sorbent particle's temperature exceeds a certain limit (which depends
on the particular sorbent) the sorbent deadburns and loses its reactivity (4).
The processes controlling capture and retention when the sorbent is maintained
under reducing conditions for a prolonged time are more complex. The principle
gas phase sulfur specie are HzS, SO2 and COS and, even though the sulfur species are
absorbed the possibility that the sulfide will decompose during burnout exists.
The data presented in Figure 6 shows a significant difference between the behavior
of coal and propane doped with H2S. This difference can be attributed to:
- With coal part of the fuel remains in the solid phase and for a given
input stoichiometry the gas phase stoichiometry in the reducing zone is
higher than with gas. Reference to Figure 1 indicates that the stability
of calcium sulfide is strongly dependent upon stoichiometry ratio;
-
With coal up to 50 percent of the sulfur remains in the solid phase under
rich conditions thus the gas phase concentration is lower than the corres-
ponding concentration with propane as the fuel;
- The conditions during burnout in the second stage will be different for the
solid and gaseous fuels and this could affect retention of the sulfur
during burnout.
These tests indicate that there is the potential to remove greater than 50
percent of the input sulfur with Ca/S molar ratios of two when coal is burned
under low NOx conditions. Further work is necessary to insure that the controlling
conditions can be achieved in practical combustors and that the sorbent injection
does not adversely impact combustor performance.
References
1.
2.
3.
4.
5.
6.
Gartrell, F.F., "Full Scale Desulfurization of Stack Gas by Dry Limestone
Injection", EPA-650/2-73-019 a, b, c, 1973.
Zallen, D.M., Gershman, R., Heap, M.P., Nurick, W.H., "The Generalization of
Low Emission Coal Burner Technology", Proceedings, Third Stationary Source
Combustion Symposium, EPA-600/7-70-050 b, 1979.
Flament, G., "The Simultaneous Reduction of NOx and SO2 in Coal Flames by
Direct Injection of Sorbents in a Staged Mixing Burner", IFRF doc. no.
G 19/a/10, 1981.
Coutant, R.W., et al., "Investigation of the Reactivity of Limestone and
Dolomite for Capturing SO2 from Flue Gas", Batelle Memorial Institute,
Final Report, EPA Contract PH-86-67-115, 1970.
Borgwardt, R.H., "Kinetic of the Reaction of SO2 with Calcined Limestone",
NAPCA, Vol. 4, p. 59, 1970.
Borgwardt, R.H., Presentation at the EPA SPO Contractors Meeting, Raleigh,
North Carolina, October 1981.
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